Project Summary Approximately 22 million Americans have a hearing impairment. While hearing devices such as hearing aids (HAs) and cochlear implants (CIs) are successful in improving speech recognition for many hearing- impaired individuals, there is still significant variability in benefit, and speech recognition in noise remains a problem. One factor that may limit benefit, especially binaural benefit, is abnormal binaural spectral integration. A prerequisite for binaural integration is binaural fusion ? the fusion of stimuli from the two ears into a single auditory object. Our findings from the previous grant showed that unlike normal-hearing (NH) listeners, many HA and CI users experience abnormally ?broad? binaural fusion in the spectral domain, such that pitches that differ greatly in frequency between the two ears are still heard as a single percept. Individuals with broad fusion also experience abnormal binaural spectral integration - averaging and thus distortion of spectral information across the ears when disparate sounds are fused. We also showed broad fusion to be associated with binaural interference ? poorer speech recognition with two ears compared to one. More importantly, preliminary data show that broad fusion is associated with greater difficulty with understanding speech in challenging multi-talker listening situations, such as noisy restaurants. Difficulties with speech understanding in noise is a major complaint of both HA and CI users. In order to help hearing-impaired listeners reduce binaural interference and improve speech understanding in background noise, we need to understand the underlying causes and factors in broad fusion. The long-term goal of this research program is to investigate the effects, causes, and potential treatments for abnormal binaural spectral integration in hearing-impaired listeners. In this proposal we will: 1) determine how broad binaural fusion affects speech perception in quiet and in background talkers; and 2) investigate potential causes of broad binaural fusion in children and adults with HAs and CIs, focusing on peripheral spectral resolution, auditory experience, and top-down auditory processing factors. The proposed research will indicate the role of broad binaural spectral fusion in difficulties faced by hearing-impaired listeners, especially for speech understanding in background noise. Determination of the factors underlying broad fusion will inform future rehabilitation approaches to treat broad fusion, and help hearing-impaired listeners attain the same benefits of binaural hearing as NH listeners.